Surface water in the North Equatorial Current (NEC) is composed of southern low-salinity water diluted by precipitation less than 34.2 psu and northern high-salinity tropical water greater than 34.8 psu. Obvious salinity front (34.5 psu) generated by the two water masses was usually located around 15'N. Effect of EI Nino/Southern Oscillation (ENSO) on salinity distribution was confined in the surface layer, while that on temperature appeared in the middle layer. Location of the salinity front sometimes moved southward largely south of 5°N and the southern oscillation index (501) was well correlated with the movemet.